The principal purpose of digital X-ray apparatus calibration is a production of highest quality images of the object under examination (patient) in course of radiological exploration. The quality of the explorative images is highly influenced by mechanical instability of the X-ray computed tomograph stand (also known as tomograph gantry) and of the C-arm and U-arm stands of the digital X-ray apparatuses. So for example, the changes in digital X-ray apparatus stand rotation path may result in failures of the preliminary calibrations of the detector, such as calibration of shading, etc. For the computed tomography apparatuses, besides the failure of the detector preliminary calibrations, even smallest changes in the path of the tomographic scanning may result in considerable deterioration of an image being reconstructed. The mechanical instability of the computed X-ray tomograph stand, of the C-arm and U-arm stands of digital X-ray apparatuses includes regular and random components. The invention under application is designed for accurate correction of the images obtained with use of X-ray apparatuses with due account of the regular component of the mechanical instability of the digital X-ray apparatus stand.
The following calibration methods of the mechanical instability of the X-ray apparatuses with the C-arm stand, in which the external monitoring systems of the apparatus mechanical instability are applied, are known:
a) It is known the method [Application for a patent USA No. 2001053204, publ. 20 Dec. 2001, IPC A61B6/00], according to which a special marker is installed onto X-ray tube and, with use of an external optical stereo camera, which is not connected to the X-ray apparatus, the rotation path of the apparatus is being tracked; the required corrections for the apparatus mechanical instability are determined regarding the path.
The obstacles to achieve the said below technical result, when using the known method, include, besides the complexity and the expensiveness of the given method application, possible obstructing of the optical stereo camera field of view (the marker becomes invisible for the camera during the apparatus rotation when the patient's table stands between the camera and the marker or when a medical staff person intrudes into the camera field of view, etc.) Thus, a part of diagnostic data will be obtained without corrections for mechanical instability of the apparatus, which circumstance leads to deterioration of the X-ray images being obtained.
b) It is known the method [U.S. Pat. No. 6,120,180, publ. 19 Sep. 2000, IPC A61B6/00], according to which ultrasonic emitters are mounted onto X-ray tube and X-ray detector, the emitters signals are registered by means of at least two ultrasonic receivers. The signals received by the ultrasonic detectors are processed and the rotation path of the apparatus is determined using them; the corrections for mechanical instability of the apparatus are determined regarding the path.
The obstacles to achieve the said below technical result, when using the known method, include, besides the complexity and the expensiveness of the given method application, the fact that the ultrasonic radiation characteristics depend on the temperature of the medium through which it propagates. This leads to necessity of complex calculations and correction of the information received by the ultrasonic data receivers, because the temperatures of X-ray tube, X-ray sensor and the ambient air are different.
It is known the calibration method for computed X-ray tomograph [U.S. Pat. No. 5,822,396, publ. 13 Oct. 1998, IPC A61B6/00], according to which test objects are placed into the scanning field for each exposure of the object under examination (a patient, when the X-ray apparatus is used in medicine). The divergences between the observed and ideal positions let calculate displacement data for each roentgenogram, which data are used for real time compensation of the stand mechanical instability.
The obstacles to achieve the said below technical result, when using the known method include the fact, that the test objects of this method are made of X-ray contrast material, of a metal as a rule. Presence of such objects during each exposure causes the formation of artifacts on the image being reconstructed; that leads to inaccuracies when X-ray tomographic examining.
The same purpose method most close to the both variants of the invention under application regarding its technical essence is calibration method for computed X-ray tomograph given in [Fahrig R., Holdsworth D. W., Three-dimensional computed tomographic reconstruction using a C-arm mounted XRII: image-based correction of gantry motion nonidealities. Medical Physics, 2000, V. 27(1), c. 30-38.]. The method includes obtaining of preliminary calibration data for regular component of the mechanical instability of X-ray apparatus stand using a steel ball (or several balls) installed in the scanning field of X-ray apparatus. The method implies obtaining of a series of N roentgenograms for different the stand rotation angles φi. The roentgenograms are obtained and processed in digital form. In each roentgenogram, the ball (or several balls) projection centre coordinates are to be found. Using the found coordinates, the regular component of the X-ray apparatus stand mechanical instability is determined. In the method, the leading role belongs to X-direction and Y-direction calibration data of X-ray apparatus stand mechanical instability, the obstacles to achieve the said below technical result using the known method include the fact, that in the known method the regular component of digital X-ray apparatus stand, Z-direction mechanical instability (scale calibration data) is not determined. The scale calibration data are relatively small in absolute values, although in case of the focus distance of 1200 mm and the object under examination size of 100 mm, the Z-direction shift by 10 mm causes change of the object under examination projection size by 1.7% (if the object under examination is located in the middle between the X-ray tube and the detector), that is inappropriate in the examinations, where high precision of measurements is needed (computerized tomography, vessels prosthetics angiography, etc.), The said Z-direction component of the stand mechanical instability (influencing the scale) may lead to error during the X-direction and Y-direction calibration data determination.